1. Field of the Invention
The present invention relates to an apparatus for monitoring the progress of membrane fouling that occurs on pores as well as on the surface of a membrane by means of varaitions of zeta potential (xcex6) of a hollow-fiber membrane measured according to time passage of filtration of a suspension, wherein colloid particles, biopolymers and other inorganic particles are dispersed, and the method thereof. Moreover, the present invention also relates to a method to identify the effect of concentration polarization layer and cake layer which can vary according to the axial position of a hollow-fiber and the subsequent developing progress of a membrane fouling by measuring the position-dependent zeta potential of the hollow-fiber membrane.
2. Description of the Related Art
In conventional methods, measurements of streaming potential of a membrane have been implemented by employing either a flat-plate type or a tubular membrane and the related studies have been largely restricted to charged property of membrane surface or electrokinetic phenomena. Therefore, there is a need for the development of a technology that can interpret the fouling progress of a given membrane via changes in zeta potential according to time passage of filtration as well as measurement of streaming potential of a hollow-fiber membrane.
Zeta potential, being defined based on electrostatic and electrokinetic principles, is known to provide useful real-time information on the surface property and the interaction between membrane and particles in actual operational situations and physicochemical conditions without incurring structural change of membrane or disturbance of flow condition. That is, zeta potential can not only provide information on electrostatic field when the membrane surface is in contact with a flowing solution but can be also an important physical quantity related to a criterion of membrane fouling resulted from adsorption or deposition of particles thus determining the property and performance of a membrane.
In the present invention, electrodes were installed both inside and outside of an inlet and an outlet of a hollow-fiber membrane, respectively, to measure the streaming potential. The difference between streaming potentials perceived simultaneously at these electrodes were used to evaluate the value of zeta potential.
The conventional apparatus and methods related to the present invention are described hereunder.
Ricq et al. [Journal of Membrane Science, 114(1996), 27-38] studied the properties of the initial virgin and the fouled membranes after filtration of a tubular inorganic membrane by measuring streaming potential and analyzing permeate flux. They installed platinum electrodes such that they penetrated the internal channel of a membrane and measured the streaming potential difference and permeate flux. The membrane used, however, was not a hollow-fiber membrane but a tubular membrane and the measurements were not made at various positions but at the inlet.
Japanese Pat. No. 62-47545 discloses a method to measure streaming potential as a way to identify the property of zeta potential inside of a hollow-shaped cylindrical tube. This method relates to the measurement of streaming potential of the internal wall of a cylindrical tube, a kind of a pipe, unlike the apparatus of the present invention which relates to a hollow-fiber having membrane pores. This method enables to measure the zeta potential of the internal wall since a given solution can flow through the cyclindrical tube, however, it cannot measure the property of membrane pores located on the radial wall of a hollow-fiber as shown in the present invention.
Japanese Published Pat. Appln. No. 11-197472 discloses a method to analyze fouling in a given separation membrane as a way to identify the fouling of a reverse osmosis membrane. This method enables to identify the fouling of a flat-plate reverse osmosis membrane by comparing the zeta potentials on membrane surface before and after the fouling and also sets up the washing conditions of the membrane. However, this method is only related to the application of the result of zeta potential to the observation of membrane fouling and is not related to the method or the apparatus of measuring streaming potential. The example 2 of the present invention also shows that the zeta potential changes according to the membrane fouling.
Szymczyk et al. conducted a study on zeta potential according to the change in ionic concentration of electrolytes by installing an Ag/AgCl electrode at each given point on both an upper and a lower region of plane inorganic membrane [Journal of Membrane Science, 134(1997), 59-66].
Japanese Published Pat. Appln. No. 8-101158 discloses a method to measure streaming potential of porous materials and Japanese Published Pat. Appln. No. 10-38836 discloses an apparatus to measure streaming potential.
These methods and apparatus, being designed for porous materials, cannot be applied to a hollow-fiber membrane and also cannot be used in measuring zeta potentials at local positions.
It is essential to provide fine installments of electrodes which carry out measurements of minute streaming potential difference in order to obtain the membrane zeta potential. A hollow-fiber membrane is not advantageous in that it has a very narrow internal diameter unlike a flat-plate or a tubular membrane, and this results in difficulty when installing internal electrodes and also becomes liable to damage the hollow-fiber or disturb the liquid flow. Moreover, the cross-flow filtration enables to generate a concentration polarization layer as the filtration is run along the axial direction and the continued permeation results in change in particle concentrations as well as the pressure drop, according to the axial position.
The present invention installed electrodes both inside and outside of an inlet and an outlet of a hollow-fiber membrane, respectively, and also provided a device to sense the minute change of streaming potential difference generated by the minute pressure difference across the membrane pores.
The present invention succeeded in monitoring the progress of membrane fouling over time by evaluating the zeta potential of a hollow-fiber membrane by continuously measuring the streaming potential in two given positions according to time passage of filtration of a given suspension.